The invention relates to an electromagnetic source for a well, forming a permanent casing element, for studying properties of the geological formations or of the fluids in the vicinity of the source.
The variations of an electromagnetic field are characteristic of the geological formations through which this field passes and of the fluids which they contain. In petroleum or mining exploration, for studying the electromagnetic properties of the geological formations through which a drill well passes, it is customary to emit an electromagnetic field from an electromagnetic source lowered into the well and located at a specific point, and to measure this field at other specific points of the well so that the intervals between the source and the receivers located at the measuring points are variable. Similar techniques involve measuring the field at points located on the surface of the ground or even along a second well. The sources conventionally consist of coils, through which circulates a current provided by an electrical supply and which are lowered into the well by means of a cable. The measurements are most often carried out after the casing of the well or wells.
Such sources have disadvantages. In particular, the casing acts as a screen, the effect of this being to limit the radius of investigation of the measuring device and making necessary, as compensation, to increase the power of the current circulating in the emitter coil. Another disadvantage is that the use of such sources requires the temporary immobilisation of the well and allows only a paintwise investigation of the formations.